Opportunistic heterotrophy in gametophytes of the homosporous fern Ceratopteris richardii

Abstract

Fern gametophytes are extremely shade-tolerant, potentially existing for long periods under conditions of extreme light limitation. Many previous studies have demonstrated an increase in gametophyte growth and incidence of spontaneous transition to sporophyte morphology (apogamy) under culture on media containing exogenous sugar. However, these studies did not verify sugar uptake or quantify relative growth on media containing different sugar types. Here, we examine the extent of heterotrophy and underlying mechanisms of sugar transport in photosynthetic gametophytes of the fern Ceratopteris richardii Brongn. Exogenous sugar uptake, growth, and sugar transport were evaluated with assays of exogenous glucose depletion, experimental culture of gametophytes under different sugar and light conditions, and bioinformatic approaches. The glucose from the growth media was significantly depleted by gametophytes growing under all conditions, especially those in the dark compared with those exposed to higher light. Gametophyte area increased similarly when cultured on equimolar concentrations of either glucose or the disaccharide sucrose, likely due to preferential uptake of one of the monomers of sucrose. Although at least one gene with similarity to sucrose transporters is expressed in germinating spores, our results suggest a reliance on monosaccharide transport for exogenous sugar uptake. Glucose assimilation in both light and dark conditions constitutes nutritional opportunism and may enhance gametophyte survival in very low light.